The USCG prescribes that a day of boating is a minimum of 4 hours. Note on the word minimum. That means 8 hours is 1 day - 24 hours is 1 day also.
If you went sailing twice in 1 day that is to be logged as 1 day of sailing.
Other questions arise around - well I was master for 1/2 of the time - how do I log that.
In all of this, use your discretion as to what is universally the right thing to do. We are trying to establish your competence as an experienced sailor - as a good and contributing crew member - or a knowledgeable and skillful master of the vessel making good and sound decisions.
Let's say you go on a weeklong adventure and you were sharing master duties with a friend. Then log half of the days as master and half as crew.